The present invention relates to a color cathode-ray tube provided with a magnetic shield member for preventing a failure, such as a color shift, caused by deviation of the orbit of an electron beam due to the presence of an external magnetic field, such as the Earth's magnetism.
In a three electron beam type color cathode-ray tube with a shadow mask provided therein, the orbit of an electron beam generally may deviate under the influence of an external magnetic field, such as the earth's magnetism. This makes an undesired fluorescent material emit light, causing an undesirable result, such as color shift.
In order to eliminate such an influence of the external magnetic field, a color cathode-ray tube with an internal magnetic shield member incorporated therein in such a manner that it extends from the shadow mask and runs along the inner wall of a funnel has been proposed.
FIGS. 4 and 5 are respectively cross-sectional and partially enlarged perspective views of a conventional color cathode-ray tube. In the figures, a tube body 1 is made up of a neck 1a, a funnel 1b and a panel 1c. An electron gun 2 is disposed in the neck 1a. A fluorescent surface 3 is made up of small pieces of fluorescent materials which emit light of red, green and blue, respectively. These fluorescent materials are mounted mosaic on the inner surface of the panel 1c.
A shadow mask 4 is disposed in opposed relation to the fluorescent surface 3. In the shadow mask 4, a predetermined array of passage-holes 6 through which an electron beam 9 passes is formed. A frame includes a side wall side 5a which opposes a skirt portion of the panel 1c, and an opposing side 5b which is directed to the electron gun 2. The periphery of the shadow mask 4 is reinforced by the frame 5 by fixing of the peripheral edge portion of the shadow mask 4 to the side wall side 5a by means of, for example, welding. A spring 7 is fixed to each of the side wall sides 5a at one end thereof. A through-hole (not shown) is formed in the other end portion of the spring 7. The spring 7, the shadow mask 4 and the frame 5 constitute a shadow mask structure 20. The shadow mask structure 20 is mounted in such a manner that the shadow mask 4 and the fluorescent surface 3 oppose each other with a predetermined gap therebetween. This is achieved by bringing a pin (not shown) implanted on the inner surface of each of the sides of the skirt portion of the panel 1c into engagement with the through-hole of the spring 7.
An internal magnetic shield member 8, called an internal magnetic shield, is a thin plate having a high magnetic permeability and shaped into the form of a frustum of pyramid which extends along the funnel 1b. A peripheral edge portion 8a of a front end of the internal magnetic shield member 8 is fixed to the opposing side 5b of the frame 5 by, for example, welding. An electron gun 9 emits an electron beam 9. The electron beam 9 emitted from the electron gun 2 is deflected and scanned within a range indicated by a dot-dashed line in FIG. 4 by means of a deflection means (not shown). The electron beam 9 which has passed through the through-hole 6 of the shadow mask 4 irradiates the fluorescent surface 3 to selectively make the fluorescent materials emit light.
The operation of the conventional color cathode-ray tube of the above-described type will now be described.
The electron beam 9 emitted from the electron gun 2 is deflected and scanned within the range indicated by the dot-dashed line in FIG. 4 by means of the deflection means. The electron beam which has passed through the through-hole 6 of the shadow mask 4 irradiates the fluorescent surface 3 to selectively make the fluorescent materials emit light.
At that time, a curve of the flight orbit of the electron beam 9, which would occur when the color cathode-ray tube is placed in an environmental magnetism, such as the Earth's magnetism, is eliminated by the internal magnetic shield member 8. That is, in the color cathode-ray tube screened by the internal magnetic shield member 8, since the environmental magnetism is weakened by shield, a curve of the flight orbit of the electron beam 9 lessens, thus lessening deviation of the position at which the electron beam 9 is incident on the fluorescent surface 3 and hence generation of color shift.
In this arrangement, when the color cathode-ray tube is placed in such a manner that it is directed to the east (hereinafter referred to as an E direction) or to the west (hereinafter referred to as a W direction), since the magnetic flux is concentrated on the internal magnetic shield member 8 and the frame 5, the space surrounded by these components is sufficiently screened from the magnetism. On the contrary, when the color cathode-ray tube is placed in such a manner that it is directed to the north (hereinafter referred to as a N direction) or to the south (hereinafter referred to as an S direction), since the internal magnetic shield member 8 is largely opened in the direction of the fluorescent surface 3, the shield effect lessens when compared with shield when the color cathode-ray tube is directed in the E or W direction. Thus, the magnetic shield effect is anisotropic in the E-W and N-S directions. It is, however, desirable for the magnetic shield mmeber to have substantially the same level of magnetic shield effect in these two directions. Conventionally, the magnetic shield member 8 having the shape of the frustum of pyramid cannot change the magnetic shield effect thereof separately in the E-W and N-S directions; rather, it is designed on the basis of experience.
Particularly, in the color cathode-ray tube of the above-described type in which the fluorescent surface 3 is made up of the fluorescent stripes which are elongated in the vertical direction, as shown in FIG. 5, the original direction in which the electron beam 9 is curved due to the presence of the environmental magnetism during the operation of the cathode-ray tube directed in the E-W direction is vertical. Thus, color shift may not occur readily. However, the use of the magnetic shield member 8 having the shape of the frustum of pyramid changes the direction of the environmental magnetism in the cathode-ray tube located in the E-W direction independently of the predetermined magnetic shield effect of the member 8. Thus, the use of the magnetic shield member 8 may have the opposite effect to what has been intended; it may bring the results worse than those obtained when no magnetic shield member 8 is used. It is therefore difficult to improve the general magnetic shield effect in a desired way.
The Earth's magnetism, which is an environmental magnetism, is composed of horizontal and vertical components. No matter which direction the color cathode-ray tube is directed during operation, the influence of the vertical component of the Earth's magnetism on the flight orbit of the electron beam 9 is fixed over a very wide region on the Earth. Thus, if design of the cathode-ray tube is performed with the usage area thereof on the Earth taken into consideration during designing, a failure, such as color shift, can greatly be alleviated. Hence, in the design of the magnetic shield member 8, it is essential to take the capability with which the horizontal component of the Earth's magnetism is screened into consideration.
The conventional color cathode-ray tube arranged in the manner described above suffers from disadvantages in that the magnetic shield effect of the magnetic shield member 8 is insufficient and in that the magnetic shield effect of the magnetic shield member cannot be changed separately in the E-W and N-S directions.
In addition, the conventional magnetic shield member is designed from experience, and thus designing thereof is a time-consuming task.